[rsixtyone]

Hey all,

In an SN1 pathway, which reacts more quickly?

2-chloropentane vs 2-iodopentane

I guess it has to do something with the size of the halogens?

[macman104]

Which halogen is a better leaving group?  The one that more readily forms the carbocation required for the SN1 mechanism, and is more stable after it has left the carbon is going to be quicker.

[rsixtyone]

What's the trend for leaving group and halogen ions?  If you go down the column, it is a better leaving group?

[macman104]

If you go down the column, it is a better leaving group?

Yes.

[agrobert]

If you go down the column, it is a better leaving group?

Yes.

This is true but not how you should rationalize leaving group ability.  Leaving group ability depends on polarizability and resonance.  Of course halides don't have resonance but can be viewed as polarizable clouds of electron.  The larger the electron cloud the more stable it is as an anion.  Which is why iodide is a better leaving group in both unimolecular and bimolecular substitution.

[rsixtyone]

Ok thanks.  Therefore, 2-iodopentane is faster via SN1?